The present invention relates to the method of producing semiconductor devices having thin film resistive elements, and applicable for analog devices etc.
Conventionally, NiCr alloy has been utilized to form general thin film resistive elements. However, since this alloy is not commonly used in semiconductor device, this alloy has not been suitably treated in the fabrication process of semiconductor devices. FIGS. 2A-2D show one example of the conventional method of forming a thin film resistive element with NiCr alloy on a semiconductor substrate. As shown in FIG. 2A, a metal oxide semiconductor (MOS) element is formed on a semiconductor substrate 21 within an area surrounded by field oxide films 22. The MOS element is comprised of a gate insulating film 23, a gate electrode 24, a drain region 25 and a source region 26. Then, an intermediate insulating film 27 is formed over the substrate 21. As shown in FIG. 2B, a thin resistive film composed of, for example, NiCr alloy is superposed on the intermediate insulating film 27 and etched to form a thin film resistive element 28. Next, as shown in FIG. 2C, an inter-layer insulating film is formed over the resistive element 28. Thereafter, the intermediate insulating film 27 and inter-layer insulating film are etched to form contact holes 29 to the thin film resistive element 28 and other contact holes 30 to the semiconductor element. However, these contact holes can not be formed concurrently in general, but must be formed separately, because the total thickness of insulating film over the thin film resistive element 28 is thinner than that of the insulating film over the semiconductor element. If dry etching were utilized to concurrently form the contact holes, the thin film resistive element 28 would be also etched after the thinner insulating film was removed therefrom. On the other hand, if wet etching were utilized to form the contact holes, the contact holes 30 to the semiconductor element would be broadened too much to thereby impare the micro structure of the semiconductor device. Therefore, the contact holes must be formed separately by two steps. Lastly as shown in FIG. 2D, an electrode pattern film 31 composed of aluminum (Al) etc. is formed on the substrate 21.
During the step of FIG. 2C, the substrate can not be subjected to the usual heating process above 800.degree. C. to carry out contact reflow, because such a heating process would cause deterioration of the NiCr alloy. Accordingly, the contact holes 30 to the semiconductor element is left deep and steep so that the electrode pattern film 31 can not properly cover step or edge portions of the contact holes 30 to thereby increase the possibility of forming an open-circuit.
As described above, the conventional method of forming the thin film resistive element is not effectively applied to the conventional semiconductor device. Such application would cause decrease in yield rate, and would not permit the micro process of semiconductor fabrication. In addition, the material of the thin film resistive element itself is not commonly used in the semiconductor process, and therefore the use of such material may affect the characteristics of semiconductor device.